This invention relates to a method and apparatus for analyzing the optic disc; and more particularly, a method for health analysis of the eye by the use of stereo photographs.
In glaucoma, optic nerve head changes precede functional visual field loss. Current clinical estimates of cup to disc ratios have been shown to be unreliable. The standard for objectively documenting optic nerve head changes is by stereo photography. Jonas et al has shown the pattern of neuroretinal rim loss occurs in sectoral sequence:
(inf.temp. greater than sup.temp. greater than temp. greater than inf.nasal greater than sup.nasal). Despite this, vertical and horizontal indices are still universally used when assessing stereo photographs.
Studies confirm that standard measures of cup/disc ratio are subject to extraordinarily high degrees of intra- and interobserver variability.
A method for analyzing the optic disc from photographs using vascular landmarks may be achieved by acquiring monochromatic or color simultaneous stereo images of the ocular fundus. Nasal, superotemporal, and inferotemporal reference points are marked on each disc margin with a mouse cursor. The position of the cup base along each of the two axes subtended between the nasal reference point and two temporal disc rim points are estimated. Alternatively, a central reference point within the disc may provide the axis for a triad of lines, one to a nasal landmark, and two as previously described toward the superotemporal and inferotemporal disc perimeter. Placement is intended to maximize as fully as possible the avascular alleyways and to subtend maximally the extant or anticipated zones of superior and inferior cupping, avoiding vasculature. The Chevron Ratio is the summed length of axes subtending the cupped portion of the disc over the sum of the axes to the disc margin. Reproducibility studies have demonstrated that computer-assisted measurements are twice as accurate as manually derived measurements with this technique, and take only seconds as opposed to minutes to calculate. Either manual or computerized Chevron ratios provide data vastly more reproducible than that typically attained with standard estimates of vertical or horizontal cup-to-disc ratios in common clinical use. A Discam (Marcher Enterprises, Hereford, UK) digital image camera and computer system has been programmed to provide intermediate measurements along each axis. These are initially entered by adjustment of the length, angle, and position of computer-generated angular lines from initially randomized cursor settings on screen over one of the stereo images. Cupping estimates are then entered along the assigned axes similarly, and may be re-entered along the very same axes during subsequent patient visits. The current system is therefore semi-automated, with the reference axes automatically generated on each successive stereo photograph in a patient series, but requiring the identification of the cup base by stereoanalysis of the images by the clinician along the constrained axes using a mouse at each visit. Fully automated systems are under development, pending the compilation of massed data on numerous discs from many clinicians so that the computer-assisted estimates of cupping will emulate those empirically determined by standard clinical acumen.